User:Abyssal/Portal:Devonian

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Introduction

Map of the Earth during the late Devonian, c. 370 Ma.

The Devonian (/dɪˈvoʊni.ən, dɛ-/ də-VOH-nee-ən, deh-) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at 419.2 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.9 Ma. It is named after Devon, South West England, where rocks from this period were first studied.

The first significant evolutionary radiation of life on land occurred during the Devonian, as free-sporing land plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants (pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revolution. The earliest land animals, predominantly arthropods such as myriapods, arachnids and hexapods, also became well-established early in this period, after beginning their colonization of land at least from the Ordovician period. (Full article...)

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Selected article on the Devonian world and its legacies

The subphylum (or phylum) Chelicerata constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders, mites, harvestmen, ticks, and Solifugae. Like all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate bauplan consists of two tagmata, the cephalothorax and the abdomen. The group is named for their chelicerae, appendages near the mouth generally used to feed. The group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel, which is the major body cavity.

Chelicerates were originally predators, but the group has diversified to use all the major feeding strategies. The guts of most modern chelicerates are too narrow for solid food, and they generally liquidize their food by grinding it with their chelicerae and pedipalps and flooding it with digestive enzymes. Most lay eggs that hatch as what look like miniature adults. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.

The chelicerata originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician period. The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are chelicerates. (see more...)

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Did you know?

Restoration of Hastocularis argus.

... that four-eyed harvestmen known as Tetrophthalmi once roamed the Earth?
...that eighty years on, scientists are still debating whether the Palæozoic fossils known as Chitinozoans (SEM image pictured) represent plants, animals or eggs?
... that an anomalocaridid, a bizarre Cambrian organism, has been found 100 million years later, in the Devonian era?
... that the extinct Gigantoproductus giganteus was the largest known species of brachiopod?
... that fossils of the extinct bivalve family Praenuculidae have been found on every continent except Antarctica?

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Life restoration of Tiktaalik roseae.
Photo credit: Nobu Tamura

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Selected article on the Devonian in human science, culture and economics

Marcellus Formation (also classified as the Marcellus Subgroup of the Hamilton Group, Marcellus Member of the Romney Formation, or simply the Marcellus Shale) is a unit of marine sedimentary rock found in eastern North America. Named for a distinctive outcrop near the village of Marcellus, New York, in the United States. The shale contains largely untapped natural gas reserves, and its proximity to the high-demand markets along the East Coast of the United States makes it an attractive target for energy development.

Stratigraphically, the Marcellus is the lowest unit of the Devonian age Hamilton Group, and is divided into several sub-units. Although black shale is the dominant lithology, it also contains lighter shales and interbedded limestone layers due to sea level variation during its deposition almost 400 million years ago. The black shale was deposited in relatively deep water devoid of oxygen, and is only sparsely fossiliferous. Most fossils are contained in the limestone members, and the fossil record in these layers provides important paleontological insights on faunal turnovers. The black shales also contain iron ore that was used in the early economic development of the region, and uranium and pyrite which are environmental hazards. The fissile shales are also easily eroded, presenting additional civil and environmental engineeringchallenges. (see more...)